1. Field of the Invention
The present invention relates to a LED (light emission diode) driver, and more particularly to a control circuit for LED lighting.
2. Description of Related Art
The LED driver is utilized to control the brightness of LED in accordance with its characteristic. The LED driver is utilized to control the current that flow through the LED. A higher current will increase intensity of the bright, but decrease the life of the LED. FIG. 1 shows a traditional LED driver. The output voltage VO of the LED driver is adjusted to provide a current ILED through a resistor 79 to LEDs 71-75. The current ILED can be shown as,
                              I          LED                =                                            V              O                        -                          V                              F                ⁢                                                                  ⁢                71                                      -            …            -                          V                              F                ⁢                                                                  ⁢                75                                                          R            79                                              (        1        )            wherein the VF71-VF75 are the forward voltage of the LEDs 71-75 respectively and R79 is the resistance of the resistor 79.
The drawback of the LED driver shown in FIG. 1 is the variation of the current ILED. The current ILED is changed in response to the change of the forward voltages of VF71-VF75. The forward voltages of VF71-VF75 are not constant due to the variation of production and operating temperature. Normally, the forward voltage of the LED is decreased in response to the increase of the operating temperature. The temperature coefficient of the LED forward voltage is around −2.5 mV/° C. If the LED is operated from −40° C. to +70° C., then the variation of the LED forward voltage will be around 0.275V. The LED current ILED is increased when the temperature is increased. If the resistor 79 is changed to a constant current source, the problem is still existed. For example, if 30 LEDs are connected in serial, the voltage drop at the current source would be greater than 8V when the temperature change from −40° C. to +70° C. It will cause a significant power loss and generate heat at the current source. The objective of the present invention is to develop a control circuit to regulate the LEDs current and the current source voltage for saving the power loss.